I ran into some heatsinks on ebay the other day, and snapped a couple of them up. Now that I have them, and before I even dream about cutting them I need to figure out what they can dissipate. (Degrees/Watt per inch, that is)

It's so cool! Pun intended, the UPS guy left them between my screen door and front door... ...they almost knocked my wife off the porch when she came home!

Anyway, The heatsinks are 25 inch long chunks, of 11 inch wide by 3/8inch base. Each has 32 fins that are 1 inch high. Anyone have a feel, or have a way to calculate the rough thermal dissipation per inch of something like this?

AAVID does not have this particular extrusion avaliable on their web site. I do not know the MFR for this part. (Similar part number on AAVID shows around a .4C/W at 10 inch lengths no forced air. I will try to get a picture, if this may help.

Have you looked at R-Theta's manual? It has extensive coverage of thermal resistance calculation. To find it, go to www.r-theta.com, select EXTRUSION on the left nav bar and in the next page click on DOWNLOAD CATALOG. The download is divided in three parts, and the first one contains all the theory and formulas.

Grease and clamp a bunch of metal cased resistors to the flat side. Do some ohms law calcs to find the voltage and resistance for the wattage you're trying to dissapate. Power it up, let it run for an hour and measure the temp in a few places. By playing with a variac, and thermometer for a while, you can get some real reliable data on those puppies. I almost bid on that same sink BTW.

If you read r-theta's white papers on themal resistance calcs, you can get a feel for what cutting them up will do.

Grease and clamp a bunch of metal cased resistors to the flat side. Do some ohms law calcs to find the voltage and resistance for the wattage you're trying to dissapate. Power it up, let it run for an hour and measure the temp in a few places

This is what I plan on doing, except that I don't want to spend much for resistors. (Althoug, I am considering building some large 4 ohm test loads... ...might be a good time to start that project up again )

I'll probably use a couple of old transistors, and build a regulator to maintain power disipation.

The problem, is that I would like to have a good feel what the disipation will be after I cut them, long before I actually make the cuts. I'll keep browsing through the web pages... ...There should be something similar on the web!

hello, i plugged the values for your heatsink into rod elliot's heatsink calculator, and it said about .2 C/W bare aluminum, and .083 C/W if you got them anodised black. however, id say that actual measurments using resistors and a thermometer would be more accurate than any equation could tell you. i have a couple heatsinks that im going to measure pretty soon, and i got a few LM35 TO220 thermometers to bolt to them. should be pretty accurate. good luck with the project.

Originally posted by psychokids hello, i plugged the values for your heatsink into rod elliot's heatsink calculator, and it said about .2 C/W bare aluminum, and .083 C/W if you got them anodised black. however, id say that actual measurments using resistors and a thermometer would be more accurate than any equation could tell you. i have a couple heatsinks that im going to measure pretty soon, and i got a few LM35 TO220 thermometers to bolt to them. should be pretty accurate. good luck with the project.

-Chris

Chris,

You'll have to let me know how the LM35's work out. I have a couple here, but have not used them. I use a non contact digital thermometer for most of my measurements. Surface emmisivity may affect the readings, but I think it gets me within a few degrees.

This project has been pushed way back on the list. House projects, and an article I'm trying to get published from an even older project! There's never enough time.

Peranders,

I think that they should be fine for what I want to use them for. I have some similar heatsink, a few more fins, and a little wider. I'm running an amplifier on that, and it seems to stay pretty cool. I've never done the actual measurements or calculations on that one eithor. I'll just use some schlep math, and figure a similar size chunk of aluminum... ...in reality, My amplifiers don't really ever get run very hard anyway. (It's just that lots and lots of heatsink looks neat )